Chapter 23

Blood Vessels

Blood Vessel Tunics• Walls of blood vessels have three layers, or tunics

1. Tunica externa (adventitia) – anchor BV to an organ • Larger blood vessels require own blood supply

– vasa vasorum in tunica externa

2. Tunica media – smooth muscle• Sympathetic input vasoconstriction • Parasympathetic input vasodilation

3. Tunica intima (interna) – endothelium (simple squamous ET lining)• Continuous with endocardium

Blood Vessel Tunics

Microscopic Comparison of Arteries and Veins

Types of Arteries• Three types of arteries:

1. Elastic arteries• Largest arteries, close to heart• Thick tunica media w/ elastic fibers

2. Muscular arteries• Medium diameter• Proportionally thicker tunica media

3. Arterioles –smallest arteries• Thin tunica media (< 6 layers)• Connect to capillaries

Capillaries

• Capillaries– Diameter slightly larger than

erythrocyte• Tunica intima only– Allows for rapid diffusion

• Form capillary beds– Blood flow regulated by

precapillary sphincters– Thoroughfare channel

bypasses bed• Site of metabolic exchange

Types of Capillaries1. Continuous – most common– continuous and complete endothelium (no physical holes)

2. Fenestrated – endothelial cells possess small “holes” – allow fluid exchange between blood and interstitial fluid

• eg. kidneys

3. Sinusoid – large gaps between endothelial cells – promotes transport of large molecules and cells to and from

blood• eg. Liver and spleen

Veins

• Drain capillaries – return blood to heart

• Pressure much lower than in arteries– Walls much thinner– Very little muscle in tunica media

• At rest, veins hold about 60% of body’s blood– function as blood reservoirs

Venules

• Smallest veins• Postcapillary venules -

smallest– Diapedesis occurs here

• Venules merge to form veins

Veins• Skeletal muscle pump moves

blood toward heart– Contraction of muscles

• Blood pressure too low to overcome gravity – valves prevent backflow and

pooling of blood in limbs– formed from tunica intima

Varicose Veins

Venous Return from the Abdomen

• Special system of circulation - hepatic portal system

• Drains blood from GI organs and shunts blood to liver– Allows for filtering

of ingested substances

Fetal-Placental Circulation

Fetal circulation bypasses the developing lungs, kidneys, and digestive tract

ttp://www.ethal.org.my/opencms/opencms/ethal/Images/MedGeneralImages/Placenta.gif

All nutritional, respiratory, and excretory needs are

met by the placenta

The exchange occurs via capillaries- mom’s and baby’s blood don’t mix.

• Fetal system has structures that are modified or cease to exist after birth

• Fetal circulatory pathway:– Oxygenated blood from placenta umbilical vein

ductus venosus (bypasses liver) inferior vena cava right atrium foramen ovale (shunt to LA) left ventricle aorta

– Some blood from RA RV pulmonary trunk ductus arteriosus (shunt to aorta)

– Aorta body umbilical arteries (now deoxygenated) placenta nutrient and gas exchange

Fetal-Placental Circulation

• Postnatal changes– Umbilical vessels constrict

and cease function– Ductus venosus becomes

ligamentum venosum– Foramen ovale becomes

fossa ovalis • Failure to close at birth =

patent foramen ovale– Ductus arteriosus becomes

ligamentum arteriosum• Failure to close at birth =

patent ductus arteriosus

Fetal-Placental Circulation